There efforts cover both basic and applied aspects of mycoplasmas and related wall-free prokaryotes, including their neurotoxins, antigens, biologic and genetic features involved in virulence, immunological interrelationships and possible role in human disease or diseases of uncertain etiology. Current work is directed to further characterization of a newly recovered mycoplasma (Mycoplasma genitalium) from unrogenital tract of patients with non-gonococcal urethritis. The organism shares some partial antigenic relationships to another pathogenic mycoplasma (M. pneumoniae), and this relationship appears to be mediated by similarities in surface components of the unique terminal attachment structure found in both mycoplasmas. Recently, we have identified antibody to the major attachment protein (P1) of M. pneumoniae in the serum of patients with acute respiratory infections with this organisms. Experimental pathogenicity studies in a variety of sub-human primates indicates higher primates (chimpanzees) are a suitable model to measure ability of M. genitalium strains to colonize the male urethra, produce bacteremia and induce specific antibody responses. Experimental intravaginal challenge of female monkeys and marmosets indicated persistent colonization and inflammatory responses in the lower genital tract. Other investigations on urogenital mycoplasmas has shown that the use of SP-4 culture medium gave over 30% more isolations of M. hominis from human urogenital specimens than conventional mycoplasma media. This new medium, developed in our laboratory, has also been used to recover a new genital mycoplasma (Mycoplasma muris) from mice, the first new species from mice in over 30 years. A comparative study to assess genetic relatedness among established species of three genera of mollicutes (wall-free prokaryotes) has provided new support for current species distinctions, based primarily upon phenotypic markers. Organisms with strict host and tissue specificity (such as some Mycoplasma species) exhibited marked genotypic homogeneity, while those species recovered from a variety of hosts and habitats (such as some Acholeplasma species) are more heterogeneous in genetic characteristics.